The rheology and deformation of the South Tibetan Detachment System as exposed at Zherger La, east-central Himalaya: Implications for exhumation of the Himalayan metamorphic core

Abstract

Deformation processes and rheological changes within the South Tibetan Detachment System (STDS) played a key role in accommodating exhumation of the Greater Himalayan Sequence (GHS). We present a study for the STDS in Zherger La, Yadong region, east-central Himalaya, to better quantify the spatial and temporal variations of the shear zone's internal deformation and rheology properties. Quartz microstructures and crystallographic preferred orientations, together with quantitative estimates of deformation temperatures, differential stresses, strain rates and viscosities are deduced from the ductile segment of the detachment zone. The mylonites that comprise the exposed shear zone preserve a range of deformation conditions that are interpreted in terms of progressive exhumation. Strain localization within the STDS is indicated by the microstructural evolution from penetrative deformation of constitutive minerals to localized deformation into quartz, which exhibit interconnected networks and high strain zones. Deformation temperatures decrease from 510 ± 40 to 405 ± 50 °C and differential stresses increase from 24 to 100 MPa structurally upwards, as indicated by two-feldspar thermometry and quartz paleopiezometry, respectively. Strain rates derived from quartz dislocation creep flow law are on the order of 10−12-10−13 s−1 and are broadly consistent with macrostructurally-derived strain rates of 1.8–5.1 × 10−13 s−1, which are calculated from previously determined exhumation rates and shear zone width. The estimated differential stresses and strain rates are further utilized to determine the variation of viscosities across the STDS. The viscosities within the STDS range at 1019–1020 Pa s. In conjunction with the published numerical modelling and analog modeling studies, the estimated viscosity data are compatible with the channel flow model for exhumation.